It is possible to fabricate intricate fluid systems with channel sizes as small as a micron. These devices can be mass-produced inexpensively and are expected to soon be in widespread use for simple analytical tests. However, in chemical analysis of turbid fluids, notably blood, filtering of the larger particles such as cells is generally required prior to analysis, especially optical analysis. In clinical laboratories this is generally accomplished by centrifugation. The centrifugal force generated is a function of distance from the center, and thus centrifugation is not effective in a small scale apparatus. In chemical laboratories membrane filters are used to separate the larger particles. This can be used in microscale apparatus, but clogging of the filters with use makes them impractical.
The greater diffusion of small particles relative to larger particles can be used to partially separate the species. Diffusion is a process which can easily be neglected at large scales, but rapidly becomes important at the microscale. Due to extremely small inertial forces in such structures, practically all flow in microstructures is laminar. This allows the movement of different layers of fluid and particles next to each other in a channel without any mixing other than diffusion. Moreover, due to the small lateral distances in such channels, diffusion is a powerful tool to separate molecules and small particles according to their diffusion coefficients, which is usually a function of their size.
The present invention exploits diffusion to provide simultaneous filtering and chemical reaction, which facilitates the elimination of preprocessing of specimens containing particulate constituents, thus reducing the sample size and analytical time required.